And now, we have a story for everyone who exercises or plans to exercise some day. 50 million of us belong to a gym, and others have machines in the house or basement. So, have you ever wondered just how accurate the equipment is about distance and calories? Abc's linsey davis brings us real answers tonight. Reporter: They crunch your call reaps, calculate your distance and tabulate your heart rate. But how accurate are these exercise machines? We headed to your neighborhood gym to find out. First up, calories. After making sure these treadmills are in tip-top shape, I ran for one mile. The machine logging 94 calories burned. But what was my body really burning? To get that answer, we visited the hospital for specl surgery in new york. Feel like thhannibal elector. This is actually a high tech oxygen analyzer that counts calories down to the decimal point. After doing the exact same warmup, I ran another mile at the same pace. It calculated I burned 75 calories. That's 25% less than the gym treadmill. Experts say that's because treadmills only take into i count your pace, weight and age, not your running form or specific body type. The best it can do is give you the best estimate, just based on generalities. Reporter: Next up, heart rate. I hopped on the elliptical to compare its monitor to the specialized one strapped to my chest. According to this machine, it says my heart rate is 1936, on the watch, it says 135. It was pretty accurate. Even when I really pushed it, testing my own limits perhaps even more than the machine. 175. 173 on the watch. And finally, distance. Experts say worn out or stretched out belts can make those miles seem extra long. So, make sure your gym tunes them up. But if a mile is generally a mile, why does it feel like I'm working so much harder when i run outside? There are many things within that mile that might dramatically effect how hard it is to maintain a certain pace. Reporter: Look here. On the treadmill, my leg is being pulled back. But outside, I'm having to push my body weight over my foot each time, meaning more effort to cover the same distance. Some might say this is sweating the small stuff, but those call reels add up, meaning, next time, I might just have to run a little faster. If you want your run inside to be just as challenging as your run outside, researchers suggest increasing the elevation on your treadmill to 1% or 2%. That will help re-create some of the wind. But all runs not created equ equally.

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